As a conventional spread spectrum receiver, an example of synchronized detection of each signal for a plurality of spread spectrum signals, in a state in which the signals are multiplexed using the same spreading code, is explained herein. It is considered that the synchronized detection for the signals is carried out in a multipath propagation environment, a transmission of signals from a plurality of transmission stations using the same spreading code, etc.
FIG. 13 is a schematic diagram of a synchronized detector in the conventional spread spectrum receiver used in above situations. The conventional spread spectrum receiver includes a matched filter 101, a cyclic integrator 102, an electric power conversion unit 103, an adder 104, an integration value storage unit 105, a forgetting coefficient multiplication unit 106, and a synchronized detection unit 107.
Since a correlation signal output from the matched filter 101 normally has a low SNIR (signal-to-noise-and-interference ratio), if correlation signals are used for synchronized detection as it is, sufficient synchronized detection characteristic cannot be achieved. To cope with the problem, the cyclic integrator 102 cyclically integrates correlation signals to improve SNIR and the synchronized detector 107 performs the synchronized detection.
After converting the outputs of the matched filter 101 into electric power, the cyclic integrator 102 performs cyclic integration; however, if the deviation of carrier wave frequency is sufficiently small, a coherent cyclic integration is often performed instead. In addition, the forgetting coefficient multiplication unit 106 may not be used in some cases.
An output of the cyclic integrator 102 is input to the synchronized detector 107 and the synchronized detector 107 detects a synchronization point of the multiplexed spread spectrum signal from the output of the cyclic integrator 102. The first detection method sorts the outputs of the cyclic integrator in descending order of amplitude and detects the synchronization point from the maximum output value in order. The second detection method sets a threshold takes a signal equal to or greater than the threshold as the synchronization point.
However, when implementing the first method on a hardware in the synchronized detector of the conventional spread spectrum receiver, the hardware disadvantageously becomes too complicated, resulting in increases of a scale of the hardware and power consumption. Furthermore, when implementing the first method on a DSP (digital signal processor), a high speed synchronized detection cannot be performed due to a decrease of processing speed, a delay caused by the interface between the hardware and the DSP, etc.
The second method of the synchronized detecting also has another disadvantage that it is difficult to control the threshold so as to secure a desired number of detected synchronization points. Namely, if the threshold is set too high, the number of detected synchronization points becomes small as compared with the number of spread spectrum signals. On the other hand, if the threshold is set too low, the probability of the erroneous detection of noise increases and a buffer provided in the synchronized detector may overflow, resulting in being unable to detect all the spread spectrum signals. Furthermore, it is difficult to control the threshold so as to make the number of detected synchronization points constant in an ordinary propagation environment in which a noise and an interference exist and a signal intensity changes.
Meanwhile, an autocorrelation waveform of the spreading code has unnecessary amplitude (side lobe) in an area away from a synchronization point (main lobe). If the synchronized detector erroneously detects the side lobe other than the main lobe, the synchronized detection characteristic of the detector deteriorates. Therefore, some methods have been conventionally used to detect only the main lobes. For example, the method disclosed in Japanese Patent Application Laid-Open No. 10-308588 calculates correlation values by taking a predetermined number of cyclic integral outputs at a certain point from the maximum value in order as main lobes, and eliminates only the side lobes from a received signal. After repeating the elimination of the side lobes by a designated number of times, a point having a maximum correlation value is set as a synchronized detection point.
With the conventional method, however, since a synchronized detection point cannot be determined unless the cancellation of side lobes is repeated by a designated number of times, there is a disadvantage in that time required for the synchronized detection increases. In addition, since it is necessary to store amplitudes and timings in descending order of amplitude so as to eliminate the side lobes, circuits implemented on the hardware disadvantageously becomes complicated, resulting in increases of a scale of the hardware and power consumption. Even when implementing the method with the DSP, a high speed synchronized detection cannot be performed due to a decrease of processing speed, delay caused by an interface between the hardware and the DSP, etc.
It is, therefore, an object of the present invention to provide a spread spectrum receiver capable of realizing a high speed synchronized detection using a small-scale circuit configuration.